Murine T, but not B, lymphocytes constitutively express a membrane receptor for adenosine nucleotides that opens a non-specific pore which admits Ca2+ and ethidium (314 daltons) but not propidium (415 daltons) ions. The kinetics, agonist and antagonist profiles and the passage of ions as a large as ethidium are characteristic of a P2z type purinoceptor. No expression of classical P2x, P2u, or P2Y type purinoceptors can be detected. The [Ca2+]i elevating activity of the ATP receptor is modulated during T cell differentiation. CD4+8+ double positive thymocytes are the least responsive. CD4-8+ single positive thymocytes, CD8+ splenic T cells, CD4+8- single positive thymocytes and CD4+ splenic T cells show increasing reactivity. Measurement of P2z expression by the rate of ethidium ion uptake correlates with the [Ca2+]i. The trimodal expression of P2z by splenic CD4+ T cells correlates with the subsets defined by CD44 and CD45RB, differentiation antigens that distinguish memory cells: P2z low cells are CD44 bright CD45RB bright; P2z intermediates are CD44 dull CD45RB intermediate; P2z highs are CD44 bright CD45RB dull. It is suggested that P2z receptor mediated signaling could be involved in the regulation of differentiation and cell death in the thymus and peripheral T lymphocytes.